Water-based multi-component coating composition and method for producing coated article

Abstract

The present invention provides an aqueous coating composition capable of forming a coating film having high hardness, excellent luster, and suppressed spray dust while being curable at low temperature. The aqueous multi-liquid coating composition comprises a first liquid containing a hydroxyl-containing resin (A) and a melamine resin (B), and a second liquid containing a hydrophilic polyisocyanate compound (D), the hydroxyl-containing resin (A) being at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2), the melamine resin (B) being dissolved in the first liquid or having an average particle size of less than 1 μm, the average total number of imino groups and methylol groups per triazine ring being greater than 1, and the hydrophilic polyisocyanate compound (D) being at least one of an ion-modified polyisocyanate compound (D1) and a nonion-modified polyisocyanate compound (D2).

Description

[Technical field]

[0001] The present invention relates to a water-based multi-component coating composition and a method for producing a coated article. [Background technology]

[0002] In recent years, consideration of the natural environment has been required in technical fields such as automobiles. Therefore, methods have been developed to lower the heating temperature and shorten the heating time during painting in order to save energy. For example, Patent Document 1 proposes an aqueous multi-liquid coating composition containing a base agent (I) containing a hydroxyl group-containing acrylic resin and a curing agent (II) containing an anionic hydrophilic group-containing polyisocyanate compound and / or a nonionic hydrophilic group-containing polyisocyanate compound. In Patent Document 1, the coating film is cured at a low temperature of 60°C. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2021-130812 A Summary of the Invention [Problem to be solved by the invention]

[0004] According to Patent Document 1, the above-mentioned aqueous multi-component coating composition is excellent in drying property, paint handling property, storage property, and weather resistance. However, the coating film obtained from the above-mentioned aqueous multi-component coating composition has low hardness, poor gloss, and may generate spray dust.

[0005] The present invention has been made in view of the above, and has an object to provide an aqueous multi-component coating composition which can be cured at low temperatures and yet can give a coating film having high hardness, excellent gloss, and suppressed spray dust. [Means for solving the problem]

[0006] In order to solve the above problems, the present invention provides the following aspects. [1] A first liquid containing a hydroxyl group-containing resin (A) and a melamine resin (B); A second liquid containing a hydrophilic polyisocyanate compound (D), the hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2); the melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average total number of imino groups and methylol groups per triazine ring exceeds 1; The hydrophilic polyisocyanate compound (D) is at least one of an ionically modified polyisocyanate compound (D1) and a nonionically modified polyisocyanate compound (D2). [2] A first liquid containing a hydroxyl group-containing resin (A), a melamine resin (B) and a polyurethane resin (C); A second liquid containing a hydrophilic polyisocyanate compound (D), the hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2); the melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average total number of imino groups and methylol groups per triazine ring exceeds 1; The hydrophilic polyisocyanate compound (D) is at least one of an ionically modified polyisocyanate compound (D1) and a nonionically modified polyisocyanate compound (D2). [3] The aqueous multi-component coating composition according to the above [1] or [2], wherein the ion-modified polyisocyanate compound (D1) contains at least an anion-modified polyisocyanate compound. [4] The aqueous multi-component coating composition according to the above [2], wherein the polyurethane resin (C) has a hydroxyl value of 30 mgKOH / g or less. [5] The aqueous multi-component coating composition according to any one of the above [1] to [4], wherein the hydroxyl-containing resin (A) has an acid value of 5 mgKOH / g or more and 70 mgKOH / g or less. [6] The water-based multi-component coating composition according to any one of the above [1] to [5], wherein the melamine resin (B) has a solubility parameter of 9 or more and 15 or less. [7] The aqueous multi-component coating composition according to any one of the above [1] to [6], wherein the solid content by mass of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the solid content by mass of the hydroxyl group-containing resin (A). [8] The aqueous multi-component coating composition according to the above [2], wherein the solid content mass of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the total solid content mass of the hydroxyl group-containing resin (A) and the polyurethane resin (C). [9] The aqueous multi-component coating composition according to any one of the above [1] to [8], wherein the equivalent ratio of the imino groups and methylol groups of the melamine resin (B) to the isocyanate groups of the hydrophilic polyisocyanate compound (D) (imino groups and methylol groups / isocyanate groups) is 0.2 or more and 1.1 or less.

[10] The water-based multi-component coating composition according to any one of the above [1] to [9], wherein the melamine resin (B) has a number average molecular weight of 300 or more and 3,000 or less.

[11] A step of applying any one of the above-mentioned aqueous multi-component coating compositions [1] to

[10] onto a substrate to form an uncured first coating film; A step of applying a clear coating composition onto the uncured first coating film to form an uncured clear coating film; and curing the uncured first coating film and the uncured clear coating film by heating at 70°C or higher and 110°C or lower.

[12] A step of applying any one of the above-mentioned aqueous multi-component coating compositions [1] to

[10] onto a substrate to form an uncured first coating film; A step of applying a second aqueous coating composition onto the uncured first coating film to form an uncured second coating film; A step of applying a clear coating composition onto the uncured second coating film to form an uncured clear coating film; and heating the uncured first coating film, the uncured second coating film, and the uncured clear coating film at a temperature of 70°C or higher and 110°C or lower to cure them.

[13] The method for producing a coated article according to

[11] or

[12] above, wherein the substrate includes a metal part and a resin part. Effect of the Invention

[0007] According to the present invention, it is possible to provide an aqueous coating composition which can be cured at low temperatures while forming a coating film which has high hardness, excellent gloss, and suppressed spray dust, and a method for producing a coated article using this aqueous coating composition. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] 1. First embodiment [Aqueous multi-component paint composition] The aqueous multi-liquid coating composition according to the first embodiment (hereinafter, sometimes simply referred to as the aqueous coating composition) comprises a first liquid containing a hydroxyl-containing resin (A) and a melamine resin (B), and a second liquid containing a hydrophilic polyisocyanate compound (D). The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2). The melamine resin (B) has an average total number of imino groups and methylol groups per triazine ring that exceeds 1. The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound and a nonion-modified polyisocyanate compound.

[0009] Melamine resins are generally used as curing agents for hydroxyl-containing resins. However, the reaction between melamine resins and hydroxyl-containing resins occurs at high temperatures (e.g., 140°C), so melamine resins are not suitable as curing agents for coating compositions that are cured at low temperatures. Therefore, highly reactive polyisocyanate compounds are usually used as curing agents for low-temperature curing coating compositions. However, the hardness of coating films using such coating compositions is likely to be low.

[0010] In the present disclosure, the melamine resin is used as a part of the main agent (corresponding to the first liquid of the present disclosure) rather than as a curing agent (corresponding to the second liquid of the present disclosure) for directly forming a crosslinked structure with the hydroxyl-containing resin. That is, the melamine resin is used as a substance that reacts with the polyisocyanate compound together with the hydroxyl-containing resin. The melamine resin and the polyisocyanate compound can react even at low temperatures. Therefore, when the aqueous coating composition according to the present disclosure is heated at low temperatures, the reaction between the hydroxyl-containing resin and the polyisocyanate compound and the reaction between the melamine resin and the polyisocyanate compound occur. The melamine resin has, on average, more than one imino group and / or methylol group per triazine ring that reacts with the isocyanate group of the polyisocyanate compound. Therefore, the reaction between the melamine resin and multiple polyisocyanate compounds bonded to the hydroxyl-containing resin may also occur. That is, the melamine resin is incorporated into the crosslinked structure of the hydroxyl-containing resin and the polyisocyanate compound, for example, a crosslinked structure of hydroxyl-containing resin-polyisocyanate compound-melamine resin-polyisocyanate compound-hydroxyl-containing resin is formed. In this way, the planar and rigid triazine ring of the melamine resin is incorporated into the crosslinked structure, and the hardness of the resulting coating film is increased.

[0011] Furthermore, it was found that spray dust is suppressed by including melamine resin as a part of the base agent. Spray dust is a convex part on the surface of a coating film formed when the coating composition that is not applied to the substrate during spray coating is scattered into the air and becomes droplets, which then adhere to the substrate. Spray dust is typically formed during the process of evaporation of the solvent contained in the droplets, and occurs when the coating film and the droplets do not mix well. Therefore, the higher the viscosity of the coating composition (droplets), the more likely the spray dust is to be formed. Melamine resins generally have a low molecular weight. Therefore, the viscosity of a coating composition containing melamine resin is kept low, and as a result, it is thought that the formation of spray dust is suppressed.

[0012] In the present disclosure, as described above, the hydrophilic polyisocyanate compound (D) is used as the curing agent. The hydrophilic polyisocyanate compound (D) has a hydrophilic group and two or more isocyanate groups in the molecule.

[0013] Since the coating film is formed by the reaction of the curing agent with the base agent, the dispersion state of the curing agent in the coating composition is reflected in the smoothness of the coating film. The hydrophilic polyisocyanate compound (D) used in the present disclosure exists in the aqueous coating composition in a micelle-like form in which the hydrophilic group portion surrounds the hydrophobic portion, and is highly dispersed. Therefore, the resulting coating film has excellent smoothness. A smooth coating film has excellent gloss.

[0014] In this specification, "isocyanate group" means an unblocked free isocyanate group. The unblocked isocyanate group enables low-temperature curing. Specifically, the hydrophilic polyisocyanate compound (D) is a water-dispersible polyisocyanate compound.

[0015] The aqueous coating composition is a multi-liquid type containing a first liquid and a second liquid. The aqueous coating composition may further contain a third liquid containing other components. The aqueous coating composition is prepared using a method commonly used by those skilled in the art. The aqueous coating composition can be prepared by mixing the first liquid and the second liquid. In another embodiment, the aqueous coating composition can be prepared by mixing the first liquid and the second liquid, and further the third liquid. Examples of the mixing method include a kneading and mixing method using a kneader or roll, and a dispersion and mixing method using a sand grind mill or disperser.

[0016] The aqueous coating composition contains water as a solvent. In the aqueous coating composition, the proportion of water in the solvent may be 50% by mass or more, may be 70% by mass or more, or may be 100% by mass.

[0017] The aqueous coating composition according to the present disclosure can be cured at low temperature. The aqueous coating composition according to the present disclosure can be cured at a temperature of, for example, 70°C or higher and 110°C or lower. The curing temperature may be 75°C or higher, or 80°C or higher. The curing temperature may be 105°C or lower, 100°C or lower, 95°C or lower, or 90°C or lower.

[0018] Hereinafter, the solid content may also be referred to as the non-volatile content. The solid content of the aqueous coating composition is, for example, all components of the aqueous coating composition excluding the solvent.

[0019] (1st liquid) The first liquid contains a hydroxyl group-containing resin (A) and a melamine resin (B).

[0020] (A) Hydroxyl-containing resin The hydroxyl-containing resin (A) is a resin (film-forming component) that is the base of the coating film. The film-forming component chemically reacts with other components contained in the aqueous paint composition to form the coating film, i.e., the resin film. The hydroxyl-containing resin (A) reacts with the hydrophilic polyisocyanate compound (D) to form a crosslinked structure. The hydroxyl-containing resin (A) allows the formation of a coating film with sufficient hardness.

[0021] The hydroxyl-containing resin (A) has one or more (typically, two or more) hydroxyl groups in one molecule. The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2).

[0022] The hydroxyl value (OHV) of the hydroxyl-containing resin (A) is, for example, 20 mgKOH / g or more and 180 mgKOH / g or less. When the hydroxyl value of the hydroxyl-containing resin (A) is 20 mgKOH / g or more, the breaking strength of the coating film is likely to be high. When the hydroxyl value of the hydroxyl-containing resin (A) is 180 mgKOH / g or less, the hydrophilization of the coating film is suppressed, and the water resistance is likely to be improved. The hydroxyl value of the hydroxyl-containing resin (A) may be 30 mgKOH / g or more, or 40 mgKOH / g or more. The hydroxyl value of the hydroxyl-containing resin (A) may be 150 mgKOH / g or less, 140 mgKOH / g or less, 100 mgKOH / g or less, or 80 mgKOH / g or less.

[0023] The glass transition temperature (Tg) of the hydroxyl-containing resin (A) is, for example, -20°C or higher and 100°C or lower. When the Tg of the hydroxyl-containing resin (A) is -20°C or higher, the breaking strength and hardness of the resulting coating film are likely to be improved. When the Tg of the hydroxyl-containing resin (A) is 100°C or lower, the quick-drying property of the aqueous coating composition is likely to be improved. The Tg of the hydroxyl-containing resin (A) may be -15°C or higher. The Tg of the hydroxyl-containing resin (A) may be 90°C or lower, 80°C or lower, 70°C or lower, or 50°C or lower.

[0024] The Tg may be calculated from the types and amounts of raw material monomers of the resin. The Tg may be measured by a differential scanning calorimeter (DSC).

[0025] The hydroxyl-containing resin (A) may have an acid value (AV) of 5 mgKOH / g or more and 70 mgKOH / g or less. When the acid value of the hydroxyl-containing resin (A) is 5 mgKOH / g or more, the hardness of the resulting coating film is likely to be improved. When the acid value of the hydroxyl-containing resin (A) is 70 mgKOH / g or less, the water resistance of the resulting coating film can be improved. The acid value of the hydroxyl-containing resin (A) may be 7 mgKOH / g or more, or 10 mgKOH / g or more. The acid value of the hydroxyl-containing resin (A) may be 60 mgKOH / g or less, 50 mgKOH / g or less, or 40 mgKOH / g or less.

[0026] The hydroxyl value and acid value are determined based on the solid content mass. The hydroxyl value and acid value can be measured by a known method described in JIS K 0070:1992. The hydroxyl value and acid value may be calculated from the blending amount of the unsaturated monomer in the raw material monomer of the target resin.

[0027] From the viewpoint of hardness, the hydroxyl-containing resin (A) may have a hydroxyl value of 20 mgKOH / g or more and 180 mgKOH / g or less, a Tg of -20°C or more and 100°C or less, and an acid value of 5 mgKOH / g or more and 70 mgKOH / g or less.

[0028] The weight average molecular weight (Mw) of the hydroxyl-containing resin (A) may be, for example, from 2,000 to 50,000. Mw is determined by a GPC method using polystyrene as the standard.

[0029] When the hydroxyl-containing resin (A) is dispersed in the first liquid, the average particle size may be, for example, 50 nm or more and 500 nm or less. When the average particle size of the hydroxyl-containing resin (A) is within the above range, the paint stability is improved and the smoothness of the resulting coating film is also improved. The average particle size of the hydroxyl-containing resin (A) may be 80 nm or more. The average particle size of the hydroxyl-containing resin (A) may be 300 nm or less.

[0030] The average particle size of the hydroxyl-containing resin (A) is measured by a laser diffraction / scattering method in accordance with JIS Z 8825. For this measurement, for example, a laser Doppler particle size analyzer can be used. A specific example is Microtrack UPA150 (manufactured by Nikkiso Co., Ltd.). The average particle size of the hydroxyl-containing resin (A) is measured using a sample diluted with ion-exchanged water to a resin solids concentration of 0.01% by mass.

[0031] (A1) Hydroxyl-containing acrylic resin The hydroxyl-containing acrylic resin (A1) can be produced, for example, by polymerizing a hydroxyl-containing α,β-ethylenically unsaturated monomer with another α,β-ethylenically unsaturated monomer by a known method. Commercially available hydroxyl-containing acrylic resins may also be used.

[0032] In one embodiment, the hydroxyl-containing acrylic resin (A1) may be an emulsion. The hydroxyl-containing acrylic resin in emulsion form (hydroxyl-containing acrylic resin emulsion) may have a multilayer structure including a core and a shell covering at least a part of the surface of the core, or may have a single layer structure.

[0033] The hydroxyl-containing acrylic resin emulsion is prepared by emulsion polymerization of the above-mentioned monomer mixture. The emulsion polymerization can be carried out by a polymerization method generally performed by those skilled in the art. Specifically, the emulsion polymerization is carried out by mixing an emulsifier in an aqueous medium containing water and, if necessary, an organic solvent such as alcohol, and dropping the above-mentioned monomer mixture and polymerization initiator under heating and stirring. The emulsion polymerization can also be carried out by mixing the monomer mixture, emulsifier and aqueous medium in advance, and dropping the emulsion mixture obtained by emulsification. The polymerization initiator, emulsifier, etc. can be any one known to those skilled in the art without any particular limitation.

[0034] Examples of hydroxyl group-containing α,β-ethylenically unsaturated monomers include hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, allyl alcohol, methallyl alcohol, and adducts of these with ε-caprolactone.

[0035] (Meth)acrylic acid includes both methacrylic acid and acrylic acid.

[0036] Examples of α,β-ethylenically unsaturated monomers other than those mentioned above include carboxylic acids such as acrylic acid, methacrylic acid, ethacrylic acid, crotonic acid, maleic acid, fumaric acid, itaconic acid, and the like, or dicarboxylic acid monoesters thereof; styrenes such as styrene and α-methylstyrene; acrylic acid esters such as methyl acrylate, ethyl acrylate, propyl acrylate, n-, i-, and t-butyl acrylate, 2-ethylhexyl acrylate, allyl acrylate, and lauryl acrylate; methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-, i-, and t-butyl methacrylate, 2-ethylhexyl methacrylate, allyl methacrylate, and lauryl methacrylate; polymerizable amide compounds such as acrylamide and methacrylamide; and other polymerizable compounds such as polymerizable aromatic compounds, polymerizable nitriles, polymerizable alkylene oxide compounds, polyfunctional vinyl compounds, polymerizable amine compounds, α-olefins, dienes, polymerizable carbonyl compounds, and polymerizable alkoxysilyl compounds.

[0037] The average particle size of the hydroxyl-containing acrylic resin emulsion may be, for example, 50 nm or more and 500 nm or less. When the average particle size of the hydroxyl-containing acrylic resin emulsion is within the above range, the paint stability is improved and the smoothness of the resulting coating film is also improved. The average particle size of the hydroxyl-containing acrylic resin emulsion may be 80 nm or more. The average particle size of the hydroxyl-containing acrylic resin emulsion may be 300 nm or less.

[0038] In one embodiment, the hydroxyl-containing acrylic resin (A1) may be water-soluble. The water-soluble hydroxyl-containing acrylic resin (hydroxyl-containing water-soluble acrylic resin) can be prepared, for example, by solution polymerization of a monomer mixture containing the above monomers.

[0039] The weight average molecular weight (Mw) of the hydroxyl-containing water-soluble acrylic resin is, for example, 4,000 or more and 50,000 or less. When the Mw of the hydroxyl-containing water-soluble acrylic resin is 4,000 or more, the hardness and weather resistance of the resulting coating film are likely to be improved. When the Mw of the hydroxyl-containing water-soluble acrylic resin is 50,000 or less, the excessive increase in viscosity of the aqueous coating composition is likely to be suppressed. The Mw of the hydroxyl-containing water-soluble acrylic resin may be 5,000 or more, or 8,000 or more. The Mw of the hydroxyl-containing water-soluble acrylic resin may be 40,000 or less, or 30,000 or less.

[0040] In one embodiment, the hydroxyl-containing resin (A) may contain a hydroxyl-containing acrylic resin emulsion and a hydroxyl-containing water-soluble acrylic resin. In this case, the ratio of the solid content mass We of the hydroxyl-containing acrylic resin emulsion to the solid content mass Ws of the hydroxyl-containing water-soluble acrylic resin may be, for example, We / Ws=8 / 1 to 1 / 4, or 7 / 1 to 1 / 3.

[0041] (A2) Hydroxyl-containing polyester resin The hydroxyl group-containing polyester resin can be obtained, for example, by polycondensation (ester reaction) of a polyhydric alcohol with a polybasic acid or an anhydride thereof. Commercially available hydroxyl group-containing polyester resins may also be used.

[0042] The polyhydric alcohol is not particularly limited, and examples thereof include ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, neopentyl glycol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, hydrogenated bisphenol A, hydroxyalkylated bisphenol A, 1,4-cyclohexanedimethanol, 2,2-dimethyl- Examples of such compounds include 3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate, 2,2,4-trimethyl-1,3-pentanediol, N,N-bis-(2-hydroxyethyl)dimethylhydantoin, polytetramethylene ether glycol, polycaprolactone polyol, glycerin, sorbitol, trimethylolethane, trimethylolpropane, trimethylolbutane, hexanetriol, pentaerythritol, dipentaerythritol, and tris-(hydroxyethyl)isocyanate. These compounds may be used alone or in combination of two or more.

[0043] The polybasic acid or its anhydride is not particularly limited, and examples thereof include phthalic acid, phthalic anhydride, tetrahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalic anhydride, methyltetrahydrophthalic acid, methyltetrahydrophthalic anhydride, hymic anhydride, trimellitic acid, trimellitic anhydride, pyromellitic acid, pyromellitic anhydride, isophthalic acid, terephthalic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, adipic acid, azelaic acid, sebacic acid, succinic acid, succinic anhydride, lactic acid, dodecenylsuccinic acid, dodecenylsuccinic anhydride, cyclohexane-1,4-dicarboxylic acid, and endo anhydride. These may be used alone or in combination of two or more.

[0044] The hydroxyl group-containing polyester resin may be modified with lactone, fats and oils or fatty acids, melamine resins, urethane resins, etc. The fats and oils or fatty acids are not particularly limited, and examples thereof include fats and oils such as castor oil, dehydrated castor oil, coconut oil, corn oil, cottonseed oil, linseed oil, perilla oil, poppy seed oil, safflower oil, soybean oil, and tung oil, and fatty acids extracted from these fats and oils.

[0045] The weight average molecular weight (Mw) of the hydroxyl-containing polyester resin (A2) is, for example, 2,000 or more and 20,000 or less. When the Mw of the hydroxyl-containing polyester resin (A2) is 2,000 or more, the hardness and weather resistance of the resulting coating film are likely to be improved. When the Mw of the hydroxyl-containing polyester resin (A2) is 20,000 or less, the excessive increase in viscosity of the aqueous coating composition is likely to be suppressed. The Mw of the hydroxyl-containing polyester resin (A2) may be 2,300 or more, or 2,500 or more. The Mw of the hydroxyl-containing polyester resin (A2) may be 10,000 or less, 9,000 or less, or 8,000 or less.

[0046] <Other hydroxyl-containing components> The first liquid may contain, as other hydroxyl group-containing components, for example, a polycarbonate polyol resin, a polyether polyol resin, or a polycaprolactone polyol resin.

[0047] The solid content of hydroxyl-containing components other than the hydroxyl-containing resin (A) in the total solid content of the hydroxyl-containing components (100% by mass) is, for example, 20% by mass or less, may be 15% by mass or less, or may be 10% by mass or less.

[0048] (B) Melamine resin The melamine resin (B) is a coating film-forming component. As described above, the melamine resin (B) reacts with the hydrophilic polyisocyanate compound (D) and is incorporated into the crosslinked structure of the hydroxyl group-containing resin (A) and the hydrophilic polyisocyanate compound (D), thereby providing a coating film with high hardness.

[0049] The melamine resin (B) may be water-soluble or water-insoluble. The water-soluble melamine resin (B) is dissolved in the first liquid. Therefore, the melamine resin (B) can react efficiently with the hydrophilic polyisocyanate compound (D). In addition, the smoothness of the resulting coating film is not easily impaired, and excellent glossiness is obtained.

[0050] The dispersion diameter of the melamine resin (B) in the first liquid and the aqueous coating composition can be regarded as the average particle diameter in a liquid having a concentration of 70% of the melamine resin (B) (the liquid component is n-butanol or isobutanol). In the liquid, the melamine resin (B) is dissolved or finely dispersed.

[0051] The average particle size of the melamine resin (B) may be 0.1 μm or less, may be 0.05 μm or less, or may be 0.01 μm or less. The average particle size of the melamine resin (B) may be less than the effective measurement range of a measuring device.

[0052] The average particle size of the melamine resin (B) is the 50% average particle size (D50) in the volumetric particle size distribution of the liquid material measured using a laser diffraction / scattering particle size distribution measuring device. An example of the particle size distribution measuring device is Microtrac UPA150 (manufactured by Nikkiso Co., Ltd.). The effective measurement range of the measuring device is 0.003 μm to 6.5 μm.

[0053] The first liquid and the aqueous coating composition may contain the melamine resin having an average particle size of 1 μm or more to the extent that the effect of the present embodiment is not impaired. The content of the large particle melamine resin is, for example, 1 part by mass or less, 0.5 parts by mass or less, or 0 part by mass per 100 parts by mass of the solid content of the hydroxyl group-containing resin (A).

[0054] Since the melamine resin (B) is dissolved or finely dispersed, its solubility parameter (SP value) can be measured by the turbidity point method. Acetone (HSP value Δg measured by Hansen is 9.77) is used as a good solvent, and hexane (SP value Δpl is 7.24) and deionized water (SP value Δph is 23.50) are used as poor solvents. The SP value of the melamine resin (B) measured by the turbidity point method using these good solvents and poor solvents may be 9 or more and 15 or less. The above SP value of the melamine resin (B) may be 9.5 or more and may be 10.0 or more. The above SP value of the melamine resin (B) may be 14.8 or less and may be 14.5 or less.

[0055] The SP value of melamine resin (B) can be determined by dissolving melamine resin (B) in a good solvent with a known SP value, and then performing turbidimetric titration with a poor solvent with a higher SP value than the good solvent and a poor solvent with a lower SP value. For methods of determining the SP value, see Reference 1: C. M. Hansen J. Paint. Tech., 39

[0505] , 104 (1967) and Reference 2: Toshikatsu Kobayashi, Color Materials, 77 [4], 188-192 (2004).

[0056] Specifically, the SP value of the melamine resin (B) is determined as follows. Measurement temperature: 20℃ Good solvent: Acetone (HSP value δg = 9.77) Poor solvent: hexane (SP value δpl = 7.24), deionized water (SP value δph = 23.50) Sample: 0.5 g of melamine resin (B) is weighed into a 100 ml beaker, 10 ml of a good solvent is added using a volumetric pipette, and the resin is dissolved using a magnetic stirrer.

[0057] (Turbidity measurement) Drop hexane onto the sample and measure the volume fraction of hexane at the point where turbidity occurs. pl Next, the SP value δ of the melamine resin (B) when hexane is used as a poor solvent is calculated by the following formula. ml Request.

[0058]

number

[0059] Separately, deionized water was dropped onto a sample prepared in the same manner, and the volume fraction of deionized water at the point where turbidity occurred was φ ph Next, the SP value δ of the melamine resin (B) when deionized water is used as a poor solvent is calculated by the following formula: mh Request.

[0060]

number

[0061] Melamine resin (B) SP value (δ poly ) is δ ml and δ mh It is the intermediate value between and is calculated by the following formula

[0062]

number

[0063] The melamine resin (B) is dissolved or finely dispersed, and therefore has almost no light diffusivity or light refraction. The liquid of the melamine resin (B) (having a concentration of 70% and containing n-butanol or isobutanol as a solvent) may have a Gardner color number of 2 or less, or 1, obtained by comparison with standard color glass.

[0064] The Gardner color scale is an objective method for evaluating the hue and transparency of chemical products. The Gardner color scale is evaluated in accordance with JIS K 0071-2:1998 (Color test method for chemical products - Part 2: Gardner color scale). The higher the Gardner color scale, the darker the color and the more opaque the product. The liquid of melamine resin (B) can be evaluated as colorless and transparent.

[0065] Table 1 shows the "Gardner standard color glass chromaticity coordinates" described in JIS K 0071-2:1998. The standard color glass number when the color of the sample matches is the Gardner color number of the sample. When the Gardner color number is 2 or less, the color coordinate x of the sample is 0.3177 or less, the color coordinate y is 0.3303 or less, and the luminous transmittance Y is 80% or more.

[0066] [Table 1]

[0067] The melamine resin (B) has a low melting point. The melting point of the melamine resin (B) may be -20°C or lower. The low melting point of the melamine resin (B) keeps the viscosity of the aqueous coating composition low, further suppresses spray dust, and further improves gloss. The melamine resin (B) may be liquid at a temperature of -20°C or higher and 70°C or lower.

[0068] The melting point of the melamine resin (B) can be conveniently evaluated, for example, by the following method. First, a mixture of the solvent and the melamine resin (B) is thinly spread on a plastic container. After heating at 70°C for 1 hour to remove the solvent, the fluidity is visually confirmed in an environment of 20°C to -20°C. For convenience, the temperature at which the fluidity disappears is regarded as the melting point of the melamine resin (B).

[0069] The melamine resin (B) has three nitrogen atoms (N) around the triazine ring (triazine nucleus). 1 ~N 3 Six substituents R 1 ~R 6 The structure in which -N 1 (R 1 )(R 2 ), -N 2 (R 3 )(R 4 ), -N 3 (R 5 )(R 6 )) is included.

[0070] The melamine resin (B) is, for example, a compound represented by the following general formula (1):

[0071] [ka] (In the formula, substituent R 1 ~R 6 each independently represents a hydrogen atom, an alkyl ether group, a methylol group, or a bonding portion to another triazine ring.

[0072] Alkyl ether (-CH2-OR 7 ) is composed of an alkyl group (R 7 The number of carbon atoms in R may be 1 to 8, or may be 1 to 4. 7 R may be linear or branched. 7 may be a methyl group, an ethyl group, a propyl group, or a butyl group.

[0073] The melamine resin (B) is generally composed of a polynuclear compound in which a plurality of triazine rings are bonded, but may be a mononuclear compound composed of one triazine ring.

[0074] As the melamine resin (B), for example, -N(-CH2-OR 7 )(-CH2OH) with methylol group; -N(-CH2-OR 7 )(H) imino group type; -N(-CH2-OR 7 )(-CH2OH) and -N(-CH2-OR 7 ) (H) is a methylol / imino group type.

[0075] However, the average value of the total number of imino groups and methylol groups per triazine ring (hereinafter, sometimes simply referred to as the average functionality) is more than 1. This makes it easier for the melamine resin (B) to be incorporated into the crosslinked structure, increasing the hardness of the coating film. The average functionality may be 1.2 or more, 1.5 or more, or 2.0 or more. The average functionality may be 4 or less, or 3.5 or less. This makes it easier for physical properties such as hardness and flexibility against metal thermal expansion to fall within the desired range.

[0076] Substituent R 1 ~R 6 This does not exclude the use of a full alkyl type melamine resin having only alkyl ether groups as the alkyl group. However, from the viewpoint of coating film properties such as flexibility, it is desirable to use a small amount of the full alkyl type melamine resin. For example, the amount of the full alkyl type melamine resin used in 100% by mass of the total solid content of the melamine resin contained in the first liquid may be 10% by mass or less, 5% by mass or less, or 0% by mass.

[0077] The number average molecular weight (Mn) of the melamine resin (B) may be 300 or more and 3000 or less. This keeps the viscosity of the aqueous coating composition low, further suppresses spray dust, and further improves gloss. The number average molecular weight of the melamine resin (B) may be 2500 or less, 2000 or less, 1500 or less, or 1300 or less. The number average molecular weight of the melamine resin (B) may be 350 or more, or 400 or more.

[0078] The solid content mass of the melamine resin (B) may be, for example, 7 parts by mass or more and 50 parts by mass or less with respect to 100 parts by mass of the solid content mass of the hydroxyl group-containing resin (A). This makes it easier for the curing reaction to proceed, and makes it easier to obtain a coating film with high hardness. The solid content mass of the melamine resin (B) may be 15 parts by mass or more, 17 parts by mass or more, or 20 parts by mass or more. The solid content mass of the melamine resin (B) may be 45 parts by mass or less, 40 parts by mass or less, or 35 parts by mass or less. In one embodiment, the solid content mass of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less with respect to 100 parts by mass of the solid content mass of the hydroxyl group-containing resin (A).

[0079] The equivalent ratio (imino group etc. / isocyanate group) between the imino group and methylol group (hereinafter sometimes referred to as imino group etc.) of the melamine resin (B) and the isocyanate group of the hydrophilic polyisocyanate compound (D) may be 0.2 or more and 1.1 or less, which facilitates the reaction between the melamine resin (B) and the hydrophilic polyisocyanate compound (D).

[0080] When the equivalent ratio (imino groups etc. / isocyanate groups) is 1.0, the imino groups and methylol groups react with the isocyanate groups in the right amount. When the equivalent ratio (imino groups etc. / isocyanate groups) is 0.2 or more, the reaction between the melamine resin (B) and the hydrophilic polyisocyanate compound (D) occurs easily. When the equivalent ratio (imino groups etc. / isocyanate groups) is 1.1 or less, the number of imino groups and methylol groups is not excessively greater than the number that reacts with the isocyanate groups in the right amount, so the reaction between the hydrophilic polyisocyanate compound (D) and the hydroxyl group-containing resin (A) also occurs easily.

[0081] The equivalent ratio (imino group etc. / isocyanate group) may be 0.3 or more, and may be 0.38 or more.The equivalent ratio (imino group etc. / isocyanate group) may be 1.0 or less, and may be 0.9 or less.

[0082] Since imino groups or methylol groups react with isocyanate groups in a 1:1 ratio, the equivalent ratio (imino groups etc. / isocyanate groups) can be obtained as the numerical ratio (Tm / Ti) of the total number Tm of imino groups and methylol groups in the melamine resin (B) contained in the coating composition to the total number Ti of isocyanate groups in the hydrophilic polyisocyanate compound (D) contained in the coating composition.

[0083] For example, the total number Tm of imino groups etc. contained in 100 g of a coating composition is calculated based on the following formula from the mass (amount) Wm of the melamine resin (B) contained in 100 g of the coating composition, the molecular weight Mm, and the total number Nm of imino groups and methylol groups per molecule. Total number Tm ( / 100 g) = total number of imino groups etc. Nm × (amount Wm / molecular weight Mm)

[0084] The total number Ti is similarly calculated from the mass (amount) Wi, molecular weight Mi, and number Ni of isocyanate groups per molecule (or NCO content Ri (%)) of the hydrophilic polyisocyanate compound (D) contained in 100 g of the coating composition, based on the following formula: Total number Ti ( / 100g) = Number of isocyanate groups Ni × (mixture amount Wi / molecular weight Mi) or Total number Ti( / 100g)=Blend amount Wi×NCO content Ri / (NCO molecular weight=42)

[0085] <Solvent> The first liquid contains water as a solvent and may further contain a water-soluble or water-miscible organic solvent, if necessary.

[0086] ≪Preparation method≫ The first liquid can be prepared by mixing the above-mentioned components by a method known to those skilled in the art, such as the same mixing method as that used for preparing the aqueous coating composition.

[0087] (2nd liquid) The second liquid contains a hydrophilic polyisocyanate compound (D). The hydrophilic polyisocyanate compound (D) is a curing agent that reacts with a hydroxyl-containing component (typically, a hydroxyl-containing resin (A)) to form a crosslinked structure, thereby curing the aqueous coating composition. The hydrophilic polyisocyanate compound (D) further reacts with a melamine resin (B). As a result, the rigid triazine ring of the melamine resin (B) is incorporated into the crosslinked structure between the hydroxyl-containing resin (A) and the hydrophilic polyisocyanate compound (D), increasing the hardness of the resulting coating film.

[0088] The equivalent ratio (NCO / OH) between the isocyanate group contained in the hydrophilic polyisocyanate compound (D) and the hydroxyl group contained in the hydroxyl group-containing component is, for example, 0.7 or more and 2.0 or less. The equivalent ratio (NCO / OH) may be 0.7 or more, or 0.8 or more. The equivalent ratio (NCO / OH) may be 2.0 or less, 1.8 or less, or 1.5 or less.

[0089] As the hydrophilic polyisocyanate compound (D), at least one of an ion-modified polyisocyanate compound (D1) and a nonion-modified polyisocyanate compound (D2) having an unblocked free isocyanate group is used. These are water-dispersible. At least the ion-modified polyisocyanate compound (D1) may be used.

[0090] (D1) Ion-modified polyisocyanate compound The ion-modified polyisocyanate compound (D1) has two or more isocyanate groups in the molecule. The ion-modified polyisocyanate compound (D1) has an unblocked free isocyanate group, which allows low-temperature curing. The ion-modified polyisocyanate compound (D1) also has an ionic group, which is a hydrophilic group. Therefore, the ion-modified polyisocyanate compound (D1) disperses well when mixed with the aqueous first liquid, improving the smoothness of the coating film.

[0091] The ion-modified polyisocyanate compound (D1) may include an anion-modified polyisocyanate compound. The anion-modified polyisocyanate compound can be obtained by modifying, for example, the polyisocyanate compound exemplified as the organic polyisocyanate compound (b1) with an anionic hydrophilic group. The modification with the anionic hydrophilic group is carried out so that two or more isocyanate groups remain in one molecule.

[0092] The anionic hydrophilic group is derived from, for example, carboxylic acid, sulfonic acid, phosphoric acid, silicic acid, sulfate ester, phosphate ester, or a metal salt or organic salt thereof. Among them, the anionic hydrophilic group may be derived from sulfonic acid.

[0093] (D2) Nonionic modified polyisocyanate compound The nonionic modified polyisocyanate compound (D2) has two or more isocyanate groups in the molecule. The nonionic modified polyisocyanate compound (D2) also has an unblocked free isocyanate group, which allows low-temperature curing. In addition, the nonionic modified polyisocyanate compound (D2) has a nonionic hydrophilic group. This allows the nonionic modified polyisocyanate compound (D2) to disperse well when mixed with the aqueous first liquid, improving the smoothness of the coating film.

[0094] The nonionic hydrophilic group is derived from a hydrophilic compound. The nonionic modified polyisocyanate compound (D2) can be obtained, for example, by modifying the polyisocyanate compound exemplified as the organic polyisocyanate compound (b1) with a hydrophilic compound.

[0095] Examples of the hydrophilic compound include hydrophilic polyols and hydrophilic polyethers. Examples of the hydrophilic polyols include ethylene glycol, glycerol, trimethylolpropane, pentaerythritol, and sorbitol.

[0096] <Other polyisocyanate compounds> The use of a non-hydrophilic polyisocyanate compound is not excluded. However, from the viewpoint of viscosity, it is desirable to use a small amount of the non-hydrophilic polyisocyanate compound. For example, the amount of the non-hydrophilic polyisocyanate compound in 100% by mass of the total solid content of the polyisocyanate compounds contained in the second liquid may be 50% by mass or less, 30% by mass or less, or 0% by mass.

[0097] The use of a blocked polyisocyanate compound in which an isocyanate group is blocked with a blocking agent is not excluded. However, from the viewpoint of curing properties at low temperatures, it is desirable to use an extremely small amount of the blocked polyisocyanate compound. The amount of the blocked polyisocyanate compound used may be 1% by mass or less, or may be 0% by mass.

[0098] Other hardeners The aqueous coating composition may contain other curing agents other than the hydrophilic polyisocyanate compound (D). Examples of other curing agents include epoxy compounds, aziridine compounds, carbodiimide compounds, and oxazoline compounds. These are used alone or in combination of two or more. The content of the other curing agents is appropriately set according to the hydroxyl group-containing resin.

[0099] <Solvent> The second liquid may contain a solvent that does not have a hydroxyl group. Examples of such a solvent include glycol ether-based organic solvents, acetate-based organic solvents, ketone-based organic solvents, and ester-based organic solvents. These may be used alone or in combination of two or more.

[0100] ≪Preparation method≫ The second liquid can be prepared by mixing the above-mentioned components by a method known to those skilled in the art. Examples of the mixing method include the same method as that for preparing the first liquid.

[0101] (Other Ingredients) The aqueous coating composition may contain pigments and additives commonly used in the coating field. The additives may be added to any of the first, second and third liquids. Examples of the pigments include color pigments, extender pigments and rust-preventive pigments. Examples of the additives include ultraviolet absorbers, hindered amine light stabilizers, antioxidants, crosslinked resin particles, leveling agents, defoamers, curing accelerators and viscosity adjusters.

[0102] [Painted items] The aqueous coating composition according to the present disclosure can be used to obtain a coated article, which has a coating film with high hardness, excellent gloss, and reduced spray dust.

[0103] In one embodiment, the coated article comprises a substrate and a multilayer coating film having a first coating film and a clear coating film laminated in this order. The first coating film is formed from the aqueous coating composition according to the present disclosure.

[0104] In another embodiment, a coated article includes a substrate and a multilayer coating film having a first coating film, a second coating film, and a clear coating film laminated in this order. The first coating film is formed from the aqueous coating composition according to the present disclosure.

[0105] (subject to be coated) Examples of the material of the substrate include metal, resin, and glass. Specific examples of the substrate include automobile bodies such as passenger cars, trucks, motorcycles, and buses, and automobile body parts, and automobile parts such as spoilers, bumpers, mirror covers, grilles, and door knobs.

[0106] Examples of metals include iron, copper, aluminum, tin, zinc, and alloys thereof (e.g., steel). Representative examples of metallic substrates include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0107] The metal substrate may be surface-treated. Examples of the surface treatment include phosphate treatment, chromate treatment, zirconium conversion treatment, and composite oxide treatment. The metal substrate may be further coated with an electrodeposition paint after the surface treatment. The electrodeposition paint may be of the cationic type or the anionic type.

[0108] Examples of resins include polyethylene resin, EVA resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), vinyl chloride resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), polycarbonate resin, acrylic resin, acrylonitrile butadiene styrene (ABS) resin, acrylonitrile styrene (AS) resin, polyamide resin, acetal resin, phenol resin, fluororesin, melamine resin, urethane resin, epoxy resin, and polyphenylene oxide (PPO). The resin substrate may be degreased.

[0109] The aqueous coating composition according to the present disclosure can be cured at low temperature, and therefore is suitable for coating on resin. The coating film obtained by the aqueous coating composition according to the present disclosure has excellent chipping resistance, and therefore is suitable for coating on metal. The substrate may include both a metal part (a part formed of metal) and a resin part (a part formed of resin). The metal part may be a steel plate.

[0110] (First coating) The first coating film is formed from the aqueous coating composition according to the present disclosure. The film thickness (dry film thickness) of the first coating film after curing is, for example, 5 μm or more and 80 μm or less. The dry film thickness of the first coating film may be 7 μm or more. The dry film thickness of the first coating film may be 50 μm or less.

[0111] The thickness of the coating film can be measured by an electromagnetic coating thickness meter (for example, SDM-miniR manufactured by SANKO Co., Ltd.) The thickness of the coating film is the average value of the thickness of the coating film at any five points.

[0112] (Second coating) The second coating film is formed by a second coating composition. The second coating composition will be described later. The second coating film may be a single layer, or may be a laminated coating film of two or more layers. The dry film thickness of each layer of the second coating film is, for example, 5 μm or more and 35 μm or less. The dry film thickness of each layer of the second coating film may be 7 μm or more. The dry film thickness of each layer of the second coating film may be 30 μm or less.

[0113] (Clear coating) The clear coating film is formed by a clear coating composition. The clear coating composition will be described later. The dry film thickness of the clear coating film is, for example, 10 μm or more and 80 μm or less. The dry film thickness of the clear coating film may be 20 μm or more. The dry film thickness of the clear coating film may be 60 μm or less.

[0114] [Manufacturing method of coated articles] In one embodiment, the coated article is produced by a method comprising the steps of applying the above-mentioned aqueous multi-component coating composition onto a substrate to form an uncured first coating film, applying a clear coating composition onto the uncured first coating film to form an uncured clear coating film, and heating the uncured first coating film and the uncured clear coating film at a temperature of 70°C or higher and 110°C or lower to cure them.

[0115] In another embodiment, the coated article is produced by a method comprising the steps of applying the aqueous coating composition onto a substrate to form an uncured first coating film, applying a second aqueous coating composition onto the uncured first coating film to form an uncured second coating film, applying a clear coating composition onto the uncured second coating film to form an uncured clear coating film, and heating the uncured first coating film, the uncured second coating film and the uncured clear coating film at a temperature of 70°C or higher and 110°C or lower to cure them.

[0116] Even at low temperatures between 70°C and 110°C, a coating film is formed that has high hardness, excellent gloss, and reduced spray dust.

[0117] Hereinafter, an explanation will be given by taking as an example a method for producing a coated article having a multi-layer coating film in which a first coating film, a second coating film and a clear coating film are laminated in this order.

[0118] (I) Step of forming an uncured first coating film The aqueous coating composition according to the present disclosure is applied onto a substrate to form an uncured first coating film. The first coating film improves adhesion between the second coating film and the substrate. The first coating film also makes the coating surface uniform, making it easier to suppress unevenness in the second coating film. As described above, the substrate may include both a metal part and a resin part.

[0119] Examples of the coating method include a roll coater method, air spray coating, airless spray coating, and rotary atomization coating. These methods may be combined with electrostatic coating. Among them, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic coating, for example, a rotary atomization electrostatic coater commonly known as "micro-microbell (μμbell)", "microbell (μbell)", "metallicbell (metabell)" or the like may be used.

[0120] After applying the aqueous coating composition, preliminary drying (also called preheating) may be performed before applying the second coating composition. This prevents the solvent contained in the aqueous coating composition from bumping during the curing process, making it easier to prevent popping. Furthermore, preliminary drying prevents the uncured first coating film and the second coating composition from mixing, making it difficult for a mixed layer to form. This can further improve the smoothness of the resulting coated article.

[0121] Examples of pre-drying include leaving the film at a temperature of 20° C. to 25° C. for 5 to 15 minutes, or heating the film at a temperature of 50° C. to 80° C. for 30 seconds to 10 minutes.

[0122] (II) Step of forming an uncured second coating film A second coating composition is applied onto the uncured first coating to form an uncured second coating. Two or more layers of uncured second coatings can be formed by applying the same or different second coating compositions two or more times. An interval of several minutes may be provided between the nth application of the second coating composition and the n+1th application of the second coating composition.

[0123] The coating method may be, for example, the same method as the coating method for the aqueous coating composition. After coating the second coating composition, preliminary drying may be carried out in the same manner as above.

[0124] (Second paint composition) The second coating composition may be water-based or solvent-based. The second coating composition may be water-based. The water-based second coating composition contains, for example, an acrylic resin emulsion, a water-soluble acrylic resin, a curing agent (typically, a melamine resin), and a polyether polyol resin. The second coating composition may further contain the above-mentioned various pigments, luster pigments, and various additives.

[0125] (III) Step of forming an uncured clear coating film A clear coating composition is applied onto the uncured second coating to form an uncured clear coating.

[0126] The coating method is not particularly limited. For example, the coating method may be the same as the coating method of the aqueous coating composition. Among them, from the viewpoint of coating efficiency, rotary atomization electrostatic coating is preferred. After coating the clear coating composition, preliminary drying may be performed in the same manner as above.

[0127] (Clear paint composition) The clear coating composition may be solvent-based, water-based, or powder-type. From the viewpoint of transparency or acid etching resistance, the solvent-based clear coating composition may contain an acrylic resin and / or a polyester resin as a coating film-forming resin, and an amino resin and / or an isocyanate as a curing agent. The solvent-based clear coating composition may also contain an acrylic resin and / or a polyester resin having a carboxylic acid and / or an epoxy group. The clear coating composition may contain the above-mentioned various pigments and additives to the extent that the transparency is not impaired.

[0128] (IV) Curing process Each uncured coating film is cured. Each coating film can be cured by heating. In this embodiment, the first coating film, the second coating film, and the clear coating film are cured at the same time.

[0129] The heating temperature is, for example, 70°C or more and 110°C or less. The heating temperature may be 75°C or more, or 80°C or more. The heating temperature may be 105°C or less, 100°C or less, 95°C or less, or 90°C or less. The heating time means the time during which the inside of the heating device reaches the target temperature and the coated object is kept at the target temperature, and does not take into account the time until the target temperature is reached. Examples of the heating device include a drying furnace that uses a heating source such as hot air, electricity, gas, or infrared rays.

[0130] The heating time may be appropriately set depending on the heating temperature. When the heating temperature is 70° C. or more and 110° C. or less, the heating time is, for example, 10 minutes or more and 60 minutes or less, and may be 15 minutes or more and 45 minutes or less.

[0131] 2. Second embodiment The second embodiment differs from the first embodiment in that the first liquid contains a polyurethane resin (C) in addition to a hydroxyl-containing resin (A) and a melamine resin (B). This different configuration will be described below. The other configurations of the second embodiment are the same as those of the first embodiment, so their description will be omitted. In the second embodiment, the configuration of the coated article and the manufacturing method of the coated article are the same as those of the first embodiment, so their description will be omitted.

[0132] (C) Polyurethane resin The polyurethane resin (C) is also a coating film-forming component. The polyurethane resin (C) increases the elasticity of the coating film and improves its chipping resistance.

[0133] The polyurethane resin (C) may have a hydroxyl value of 30 mgKOH / g or less. This allows the viscosity of the aqueous coating composition to be kept low. Therefore, spray dust is further suppressed and gloss is further improved. Furthermore, the reaction between the polyurethane resin (C) and the hydrophilic polyisocyanate compound (D) is suppressed, making it easier to control the progress of the reaction between the hydroxyl-containing resin (A) and the hydrophilic polyisocyanate compound (D). The hydroxyl value of the polyurethane resin (C) may be 20 mgKOH / g or less, 10 mgKOH / g or less, or 0 mgKOH / g.

[0134] In one embodiment, the solid content mass of the polyurethane resin (C) may be 10 parts by mass or more and 110 parts by mass or less per 100 parts by mass of the solid content of the hydroxyl group-containing resin (A). This makes it easier to obtain a coating film having a good balance between hardness and elasticity, which has chipping resistance and good coating strength. The solid content mass of the polyurethane resin (C) may be 15 parts by mass or more, or 20 parts by mass or more. The solid content mass of the polyurethane resin (C) may be 105 parts by mass or less, or 50 parts by mass or less.

[0135] In the first liquid, the polyurethane resin (C) may be dissolved. That is, the polyurethane resin (C) may be a water-soluble polyurethane resin. In the first liquid, the polyurethane resin (C) may be in the form of a dispersion.

[0136] The water-soluble polyurethane resin and polyurethane resin dispersion can be obtained, for example, by a method of forcibly emulsifying a polyurethane resin using a surfactant, or by a method of neutralizing a polyurethane resin with a base or an acid.

[0137] The polyurethane resin is obtained, for example, by reacting a polyol compound, a compound having an active hydrogen group and a hydrophilic group in the molecule, an organic polyisocyanate compound (b1), and, if necessary, a chain extender and a polymerization terminator. If necessary, the chain extender and the polymerization terminator may be used.

[0138] The polyol compound contains two or more hydroxyl groups in the molecule. Examples of the polyol compound include polyhydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, trimethylolpropane, and glycerin, polyether polyols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol, polyester polyols obtained from dicarboxylic acids such as adipic acid, sebacic acid, itaconic acid, maleic anhydride, phthalic acid, and isophthalic acid and glycols such as ethylene glycol, triethylene glycol, propylene glycol, butylene glycol, tripropylene glycol, and neopentyl glycol, polycaprolactone polyols, polybutadiene polyols, polycarbonate polyols, and polythioether polyols. These may be used alone or in combination of two or more.

[0139] Examples of compounds having active hydrogen groups and hydrophilic groups in the molecule include compounds containing active hydrogen and anionic, cationic or nonionic hydrophilic groups. The anionic groups include anionic groups and anion-forming groups. The anion-forming groups are groups that can react with a base to form an anionic group, and specifically, the anionic groups are formed by neutralizing with a base before, during or after the urethane reaction.

[0140] Compounds containing active hydrogen and anionic groups are described, for example, in JP-B-42-24192 and JP-B-55-41607, and specifically include α,α-dimethylolpropionic acid and α,α-dimethylolbutyric acid. Compounds having active hydrogen and cationic groups are described, for example, in JP-B-43-9076. Compounds having active hydrogen and nonionic hydrophilic groups are described, for example, in JP-B-48-41718, and specifically include polyethylene glycol and alkyl alcohol alkylene oxide adducts.

[0141] Examples of the organic polyisocyanate compound (b1) include aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, and araliphatic polyisocyanates.

[0142] Aromatic polyisocyanates have two or more isocyanate groups bonded to carbon atoms constituting an aromatic ring. Examples of aromatic polyisocyanates include aromatic diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4'-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, 2,4'- or 4,4'-diphenylmethane diisocyanate, or mixtures thereof, 2,4- or 2,6-tolylene diisocyanate, or mixtures thereof, 4,4'-toluidine diisocyanate, and 4,4'-diphenylether diisocyanate; aromatic triisocyanates such as triphenylmethane-4,4',4''-triisocyanate, 1,3,5-triisocyanatobenzene, and 2,4,6-triisocyanatotoluene; and aromatic tetraisocyanates such as 4,4'-diphenylmethane-2,2',5,5'-tetraisocyanate. These may be used alone or in combination of two or more.

[0143] Aliphatic polyisocyanates do not have an aromatic ring and have two or more isocyanate groups bonded to carbon atoms constituting a linear or branched aliphatic hydrocarbon group. Examples of aliphatic polyisocyanates include ethylene diisocyanate, trimethylene diisocyanate, 1,2-propylene diisocyanate, butylene diisocyanate (tetramethylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate), 1,5-pentamethylene diisocyanate (PDI), 1,6-hexamethylene diisocyanate (HDI), 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, 1,2-dimethylphenyl diisocyanate, 1,3-dimethylphenyl diisocyanate, 1,5-dimethylphenyl diisocyanate, 1,6 ...5-dimethylphenyl diisocyanate aliphatic diisocyanates such as ethyl ester triisocyanate, heptamethylene diisocyanate, octamethylene diisocyanate, and dodecamethylene diisocyanate; and aliphatic triisocyanates such as lysine ester triisocyanate, 1,4,8-triisocyanatooctane, 1,6,11-triisocyanatoundecane, 1,8-diisocyanato-4-isocyanatomethyloctane, 1,3,6-triisocyanatohexane, and 2,5,7-trimethyl-1,8-diisocyanato-5-isocyanatomethyloctane. These may be used alone or in combination of two or more.

[0144] Alicyclic polyisocyanates do not have an aromatic ring, but have two or more isocyanate groups bonded to carbon atoms constituting a cyclic aliphatic hydrocarbon group. Examples of alicyclic polyisocyanates include alicyclic diisocyanates such as 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (common name: isophorone diisocyanate), methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, 1,3- or 1,4-bis(isocyanatomethyl)cyclohexane (common name: hydrogenated xylylene diisocyanate), or mixtures thereof, and norbornane diisocyanate; 1,3,5-triisocyanatocyclohexane, 1,3,5-trimethylisocyanatocyclohexane, 2-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo[2.2.1 ]heptane, 2-(3-isocyanatopropyl)-2,6-di(isocyanatomethyl)-bicyclo[2.2.1]heptane, 3-(3-isocyanatopropyl)-2,5-di(isocyanatomethyl)-bicyclo[2.2.1]heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, 6-(2-isocyanatoethyl)-2-isocyanatoethyl Alicyclic triisocyanates such as isocyanatomethyl-3-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, 5-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane, and 6-(2-isocyanatoethyl)-2-isocyanatomethyl-2-(3-isocyanatopropyl)-bicyclo[2.2.1]heptane are included. These can be used alone or in combination of two or more.

[0145] Aromatic aliphatic polyisocyanates have an aromatic ring and two or more isocyanate groups bonded to carbon atoms constituting an aliphatic hydrocarbon group. Examples of aromatic aliphatic polyisocyanates include aromatic aliphatic diisocyanates such as 1,3- or 1,4-xylylene diisocyanate, or mixtures thereof, ω,ω'-diisocyanato-1,4-diethylbenzene, 1,3- or 1,4-bis(1-isocyanato-1-methylethyl)benzene (common name: tetramethylxylylene diisocyanate), or mixtures thereof; and aromatic aliphatic triisocyanates such as 1,3,5-triisocyanatomethylbenzene. These may be used alone or in combination of two or more.

[0146] The organic polyisocyanate compound (b1) may be a derivative of each of the above polyisocyanates. Examples of the derivative of polyisocyanate include dimer, trimer, biuret, allophanate, uretdione, uretoimine, isocyanurate, oxadiazinetrione, polymethylene polyphenyl polyisocyanate (crude MDI, polymeric MDI), and crude TDI.

[0147] The chain extender contains two or more active hydrogen groups in the molecule. Examples of the chain extender include low molecular weight polyols such as ethylene glycol, propylene glycol, 1,4-butanediol, 3-methylpentanediol, 2-ethyl-1,3-hexanediol, and trimethylolpropane; polyamines such as ethylenediamine, hexamethylenediamine, diethylenetriamine, hydrazine, xylylenediamine, and isophoronediamine; and water. These may be used alone or in combination of two or more.

[0148] Examples of the polymerization terminator include compounds having one active hydrogen in the molecule (such as monoalcohols and monoamines) and monoisocyanate compounds.

[0149] The polyurethane resin may be synthesized by a one-shot method in which each component is reacted at once, or by a multi-stage method in which each component is reacted stepwise. In the multi-stage method, a part of the active hydrogen-containing compound (e.g., a high molecular weight polyol) is reacted with an organic polyisocyanate compound (b1) to form an NCO-terminated prepolymer, and then the prepolymer is reacted with the remaining part of the active hydrogen-containing compound.

[0150] In this embodiment, the solid content mass of the melamine resin (B) may be 7 parts by mass or more and 50 parts by mass or less relative to 100 parts by mass of the total solid content mass of the hydroxyl group-containing resin (A) and the polyurethane resin (C). This makes it easier for the curing reaction to proceed, and the coating film properties such as hardness and flexibility against metal thermal expansion tend to be good. The solid content mass of the melamine resin (B) in this embodiment may be 15 parts by mass or more, 17 parts by mass or more, or 20 parts by mass or more. The solid content mass of the melamine resin (B) in this embodiment may be 45 parts by mass or less, 40 parts by mass or less, or 35 parts by mass or less. In particular, the solid content mass of the melamine resin (B) relative to 100 parts by mass of the total solid content mass of the hydroxyl group-containing resin (A) and the polyurethane resin (C) may be 7 parts by mass or more and 40 parts by mass or less. EXAMPLES

[0151] The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto. In the examples, "parts" and "%" are based on the mass of the solid content unless otherwise specified.

[0152] The acid value and hydroxyl value were calculated based on the solid content acid value and solid content hydroxyl value of the monomer mixture used.

[0153] [Production Example 1-1] Production of Hydroxyl-Containing Acrylic Resin (A1-1) 126.5 parts of deionized water was added to the reaction vessel, and the temperature was raised to 80°C while mixing and stirring in a nitrogen stream. Next, a monomer emulsion consisting of 30 parts of a monomer mixture (containing 6.98 parts of ethyl acrylate, 12.42 parts of butyl acrylate, 4.75 parts of styrene, 2.78 parts of 2-hydroxyethyl methacrylate, and 3.07 parts of methacrylic acid), 0.33 parts of Adeka Reasoap SR-10 (polyoxyethylene-1-alkoxymethyl-2-(2-propenyloxy)ethyl ether sulfate ester ammonium salt, manufactured by ADEKA Corporation), and 24 parts of deionized water, and an initiator solution consisting of 0.09 parts of ammonium persulfate and 3 parts of deionized water were dropped into the reaction vessel in parallel over 1.5 hours. After the dropwise addition was completed, the mixture was aged at the same temperature for 1 hour.

[0154] Next, a monomer emulsion consisting of 70 parts of a monomer mixture (containing 16.28 parts of ethyl acrylate, 28.97 parts of butyl acrylate, 15.25 parts of styrene, 6.5 parts of 2-hydroxyethyl methacrylate, and 3 parts of allyl methacrylate), 0.77 parts of Adeka Reasoap SR-10, and 56 parts of deionized water, and an initiator solution consisting of 0.21 parts of ammonium persulfate and 7 parts of deionized water were added dropwise to the reaction vessel in parallel over 1.5 hours. After the addition was completed, the mixture was aged at the same temperature for 1 hour.

[0155] The mixture was then cooled to 40°C and filtered through a 400 mesh filter. 20 parts of deionized water and 0.32 parts of dimethylaminoethanol were added to the filtrate to adjust the pH to 6.5. This resulted in an emulsion of a hydroxyl-containing acrylic resin (A1-1) having a core-shell multilayer structure with an average particle size of 100 nm, Tg of 27°C, non-volatile content of 30%, acid value of 20 mgKOH / g, and hydroxyl value of 40 mgKOH / g.

[0156] [Production Example 1-2] Production of Hydroxyl-Containing Acrylic Resin (A1-2) 126.5 parts of deionized water was added to the reaction vessel, and the temperature was raised to 80°C while mixing and stirring in a nitrogen stream. Next, 100 parts of the monomer mixture (containing 27.61 parts of methyl acrylate, 53.04 parts of ethyl acrylate, 4.00 parts of styrene, 9.28 parts of 2-hydroxyethyl methacrylate, 3.07 parts of methacrylic acid, and 3.00 parts of allyl methacrylate), a monomer emulsion consisting of 1.1 parts of Adeka Reasoap SR-10 and 80 parts of deionized water, and an initiator solution consisting of 0.3 parts of ammonium persulfate and 10 parts of deionized water were dropped into the reaction vessel in parallel over 2 hours. After the dropwise addition was completed, the mixture was aged at the same temperature for 2 hours.

[0157] The mixture was then cooled to 40°C and filtered through a 400 mesh filter. 20 parts of deionized water and 0.32 parts of dimethylaminoethanol were added to the filtrate to adjust the pH to 6.5. This resulted in an emulsion of a single-layer type hydroxyl-containing acrylic resin (A1-2) with an average particle size of 90 nm, Tg-9.5°C, non-volatile content of 30%, acid value of 20 mgKOH / g, and hydroxyl value of 40 mgKOH / g.

[0158] [Production Example 1-3] Production of Hydroxyl-Containing Acrylic Resin (A1-3) 23.89 parts of tripropylene glycol methyl ether and 16.11 parts of propylene glycol methyl ether were added to the reaction vessel, and the mixture was heated to 105°C while being mixed and stirred in a nitrogen stream. Next, a monomer mixture containing 13.1 parts of methyl methacrylate, 68.4 parts of ethyl acrylate, 11.6 parts of 2-hydroxyethyl methacrylate, and 6.9 parts of methacrylic acid was prepared, and 100 parts of the monomer mixture and an initiator solution consisting of 10.0 parts of tripropylene glycol methyl ether and 1 part of t-butylperoxy 2-ethylhexanoate were dropped into the reaction vessel in parallel over 3 hours. After the dropwise addition, the mixture was aged at the same temperature for 0.5 hours.

[0159] Further, an initiator solution consisting of 5.0 parts of tripropylene glycol methyl ether and 0.3 parts of t-butyl peroxy 2-ethylhexanoate was added dropwise to the reaction vessel over 0.5 hours. After the addition was completed, the mixture was aged at the same temperature for 2 hours.

[0160] Next, 16.1 parts of the solvent were distilled off under reduced pressure (70 torr) at 110°C using a solvent remover, and then 204 parts of deionized water and 7.1 parts of dimethylaminoethanol were added. This produced a hydroxyl-containing water-soluble acrylic resin solution (A1-3) with a non-volatile content of 30%, an acid value of 40 mgKOH / g, a hydroxyl value of 50 mgKOH / g, a Tg of 10°C, and an Mw of 30,000.

[0161] [Production Example 2] Production of Hydroxyl-Containing Polyester Resin (A2) A reaction vessel equipped with a stirrer, a nitrogen inlet tube, a temperature control device, a condenser, and a decanter was charged with 250 parts of trimethylolpropane, 824 parts of adipic acid, and 635 parts of cyclohexanedicarboxylic acid, and the temperature was raised to 180°C, and a condensation reaction was carried out until water was no longer distilled. After cooling to 60°C, 120 parts of phthalic anhydride was added. Next, the temperature was raised to 140°C and kept at this temperature for 60 minutes to obtain a polyester resin having a number average molecular weight of 2,000 as measured by GPC. 59 parts of dimethylaminoethanol (equivalent to 80% of the acid value of the resin (neutralization rate 80%)) was added at 80°C, and 1920 parts of deionized water were added and stirred to obtain a polyester water dispersion with a solid content of 45% by mass. The hydroxyl value of the hydroxyl-containing polyester resin (A2) contained in this polyester water dispersion was 110 mgKOH / g, the acid value was 15 mgKOH / g, the Tg was -14°C, and the Mw was 7,000.

[0162] [Preparation of pigment dispersion paste] 9.2 parts of dispersant "Disperbyk 190" (manufactured by BYK-Chemie), 17.8 parts of ion-exchanged water, and 73.0 parts of rutile-type titanium dioxide were premixed. Then, the mixture was mixed and dispersed at room temperature in a paint conditioner using a bead medium until the particle size became 5 μm or less. Finally, the bead medium was removed by filtration to obtain a color pigment paste.

[0163] Details of the melamine resin (B), polyurethane resin (C) and hydrophilic polyisocyanate compound (D) used are as follows:

[0164] [Melamine resin (B)] The melamine resins (B-1) to (B-7) and (b-1) to (b-3) having the following average functional groups, average particle size and number average molecular weight were used. Details are shown in Table 2.

[0165] The average particle size of the melamine resin (B) was measured using a liquid with a concentration of 70% of melamine resin (B) using n-butanol or isobutanol. The volumetric particle size distribution of the liquid was measured using a laser diffraction / scattering particle size distribution measuring device (Microtrac UPA150, manufactured by Nikkiso Co., Ltd.), and the 50% average particle size (D50) in the particle size distribution was taken as the average particle size of the melamine resin (B). The effective measurement range of the Microtrac UPA150 is 0.003 μm to 6.5 μm. In Table 2, the description that D50 is less than 0.003 μm indicates that D50 was less than the effective measurement range of the measuring device.

[0166] [Table 2]

[0167] [Polyurethane resin (C)] (C-1): N800T, Sanyo Chemical Industries, Ltd., hydroxyl value 0 mg KOH / g (C-2): Bayhydrol UH2648 / 1, manufactured by Covestro, water-based polyurethane resin, hydroxyl value 0 mg KOH / g (C-3): Bayhydrol U2787, manufactured by Covestro, water-based polyurethane resin, hydroxyl value 56 mg KOH / g

[0168] [Ion (anion) modified polyisocyanate compound (D1)] (D1): Bayhydur 2655, manufactured by Sumika Covestro Urethane Co., Ltd., containing sulfonic acid groups [Nonionic modified polyisocyanate compound (D2)] (D2): Bayhydur 304, manufactured by Sumika Covestro Urethane Co., Ltd., hydrophilic polyether modified HDI trimmer

[0169] [Blocked polyisocyanate compounds] (d1): Duranate WM44-L70G, manufactured by Asahi Kasei Corporation, HDI-based blocked isocyanate [Non-hydrophilic polyisocyanate compounds] (d2): Sumidur N3300, manufactured by Sumika Covestro Urethane Co., Ltd., urethodione derivative of hexamethylene diisocyanate

[0170] Examples 1 to 12 corresponding to the first embodiment were carried out. [Example 1] (1) Preparation of first solution In a container equipped with a stirrer, 35 parts of hydroxyl-containing acrylic resin (A1-1), 10 parts of hydroxyl-containing polyester resin (A2), 10 parts of melamine resin (B-1), 122.87 parts of color pigment paste, and 25 parts of ion-exchanged water were added, and the pH was adjusted to 8.0 with 0.01 parts of dimethylethanolamine (Kishida Chemical Co., Ltd.). Furthermore, 1.0 parts of Adekanol UH-814N (urethane association type viscosity agent, active ingredient 30%, Asahi Denka Kogyo Co., Ltd., trade name) were mixed and stirred. Then, Typec CR-97 (Ishihara Sangyo Kaisha, titanium dioxide, primary average particle size 200 nm) was added so that the PWC was 53.5% and dispersed. This resulted in the first liquid being obtained.

[0171] (2) Preparation of second liquid 45 parts of the anion-modified polyisocyanate compound (D1) and an appropriate amount of a solvent (dipropylene glycol dimethyl ether and / or ethylene glycol monobutyl acetate) were mixed and thoroughly stirred with a disper to obtain a second liquid.

[0172] (3) Preparation of Water-Based Paint Composition The first and second liquids were mixed to obtain an aqueous coating composition.

[0173] (4) Preparation of the object to be coated (metal coated object) A matte steel plate (400mm x 600mm) was used as the substrate, and was washed and zinc phosphate treated according to the usual method. It was then electrocoated using a cationic electrocoating paint (Power Top U-100, manufactured by Nippon Paint Co., Ltd.). It was then dried at 170°C for 20 minutes to obtain a metal substrate with an electrocoating film of 15μm thickness.

[0174] (Resin coated object) A polypropylene plate was degreased to obtain a resin substrate.

[0175] (5) Preparation of painted articles with multi-layer coatings The aqueous multi-component coating composition was electrostatically coated onto each of the metal and resin substrates using a rotary atomizer type electrostatic coater to give a dry film thickness of 15 μm, and the coating was allowed to stand for 5 minutes. Next, an aqueous base paint composition (Nippon Paint Automotive Coatings, aqueous base AR-3020-1 (gray metallic)) was applied using a rotary atomizer electrostatic coater to a dry film thickness of 15 μm, and the substrate was preheated at 80°C for 3 minutes. Next, a clear paint (PolyureExcel O-1200 (product name), a two-component acrylic urethane organic solvent-based clear paint containing a polyisocyanate compound, manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was applied using a rotary atomizing electrostatic coater so that the dry film thickness was 35 μm. Thereafter, the coated article was heated at 80° C. for 20 minutes to obtain a coated article having a multi-layer coating film.

[0176] [Examples 2 to 12 and Comparative Examples 1 to 4 and 6] Aqueous paint compositions were prepared and coated articles were obtained in the same manner as in Example 1, except that the types and amounts of the components and the solid content mass at the time of coating were changed as shown in Tables 3 and 4.

[0177] [Comparative Example 5] An aqueous coating composition was prepared in the same manner as in Example 1, except that the melamine resin (B-1) was added to the second liquid instead of the first liquid. When this aqueous coating composition was allowed to stand at 23°C for 24 hours, the viscosity of the coating composition increased significantly and it was no longer possible to spray coat it, and therefore the following evaluations could not be carried out.

[0178] [Comparative Example 7] An aqueous coating composition was prepared in the same manner as in Example 1, except that in preparing the first liquid, 10 parts of melamine resin particles (product name: Eposter MS, manufactured by Nippon Shokubai Co., Ltd.) having an average particle size of 2 μm were added instead of 10 parts of the melamine resin (B-1).

[0179] [evaluation] The aqueous coating compositions prepared in the above Examples and Comparative Examples were used to carry out the following evaluations. In Comparative Example 7, hardness and gloss were evaluated.

[0180] (1) Hardness of single layer coating The aqueous multi-liquid coating composition was electrostatically coated on a glass plate using a rotary atomizing electrostatic coater so that the dry film thickness was 15 μm, and the plate was left for 5 minutes. The plate was then heated at 85°C for 20 minutes (keep time) to obtain a test plate A for evaluation having a single-layer coating film. The test plate A for evaluation was set in a pendulum hardness tester (manufactured by BYK Co., Ltd.) to measure the number of vibrations required for the swing angle of the Konig pendulum at 23°C to decrease from 6 degrees to 3 degrees, and evaluated according to the following evaluation criteria.

[0181] (Evaluation Criteria) A Vibration frequency is 80 or more B: Vibration frequency is between 65 and 80 C Vibration frequency is 50 to less than 65 times D. Vibration frequency is between 35 and 50 times E Vibration frequency is less than 35 times

[0182] (2) Glossy finish of single-layer coating The aqueous multi-liquid coating composition was electrostatically coated on a metal substrate prepared in the same manner as above using a rotary atomizing electrostatic coater so that the dry film thickness was 15 μm, and the substrate was left for 5 minutes. The substrate was then heated at 85° C. for 20 minutes (holding time) to obtain a test panel B for evaluation having a single-layer coating film.

[0183] The gloss value of the evaluation test plate B was measured in accordance with JIS K5600-4-7 using "MULTIGLOSS268plus" (manufactured by Konica Minolta, Inc.). In detail, the gloss value of the evaluation test plate B was measured three times under the geometric condition of an incident optical axis of 60° (60° gloss). The average value of the obtained measured values ​​was calculated and evaluated according to the following criteria.

[0184] A 60° gloss value is 70 or more B 60° gloss value is 60 or more and less than 70 C 60° gloss value is 50 or more and less than 60 D 60° gloss value is 40 or more and less than 50 E 60° gloss value less than 40

[0185] (3) Spray dust A water-based coating composition was applied by air spray coating under the conditions below to a metal substrate prepared in the same manner as above, to obtain a test panel C for evaluation.

[0186] (Air spray painting conditions) Discharge: 2 turns back from full close Air pressure: 0.25MPa

[0187] (Painting method) Coating was carried out on a metal substrate held substantially vertically in the following manner. Step 1. Apply the water-based multi-component paint composition to the object to be coated (gun distance 30 cm) so that the dry film thickness is 15 μm. Step 2. Dry at room temperature for 10 minutes Step 3. Stand at a 45° angle from the object to be coated, 150 cm away from the gun, and spray dust for 10 seconds. Step 4. Let dry at room temperature for 10 seconds Step 5. Repeat steps 3 and 4 a total of five times. Step 9. Dry in a jet oven at 60℃ for 1 hour

[0188] The coating film of the obtained evaluation test plate C was visually observed from above, and the number of spray dust particles adhering to the test plate (30 cm x 40 cm) was evaluated according to the following criteria: A mound having a diameter of 100 μm or more was regarded as spray dust. 5. Less than 10 4. 10 or more but less than 30 3. 30 or more but less than 100 2. 100 or more but less than 200 1 200 pieces or more

[0189] [Table 3]

[0190] [Table 4]

[0191] Examples 13 to 31 corresponding to the second embodiment were carried out. [Example 13] (1) Preparation of first solution In a container equipped with a stirrer, 40 parts of hydroxyl-containing acrylic resin (A1-1), 10 parts of hydroxyl-containing polyester resin (A2), 15 parts of melamine resin (B-1), 10 parts of polyurethane resin (C-1), 122.11 parts of color pigment paste, and 68 parts of ion-exchanged water were added, and the pH was adjusted to 8.0 with 0.01 parts of dimethylethanolamine (Kishida Chemical Co., Ltd.). Furthermore, 1.0 parts of Adekanol UH-814N (urethane association type viscosity agent, active ingredient 30%, Asahi Denka Kogyo Co., Ltd., trade name) were mixed and stirred. Then, Typec CR-97 (Ishihara Sangyo Co., Ltd., titanium dioxide, primary average particle size 200 nm) was added so that the PWC was 53.5%, and dispersed. This resulted in the first liquid being obtained.

[0192] (2) Preparation of second liquid 25 parts of the anion-modified polyisocyanate compound (D1) and an appropriate amount of a solvent (dipropylene glycol dimethyl ether and / or ethylene glycol monobutyl acetate) were mixed and thoroughly stirred with a disper to obtain a second liquid.

[0193] An aqueous coating composition and a coated article were obtained in the same manner as in Example 1, except that the above first and second liquids were used.

[0194] [Examples 14 to 31 and Comparative Examples 8 to 11 and 13] Aqueous coating compositions were prepared and coated articles were obtained in the same manner as in Example 13, except that the types and amounts of the components and the solid content mass at the time of coating were changed as shown in Tables 5 to 7.

[0195] [Comparative Example 12] An aqueous coating composition was prepared in the same manner as in Example 13, except that the melamine resin (B-1) was added to the second liquid instead of the first liquid. When this aqueous coating composition was allowed to stand at 23°C for 24 hours, the viscosity of the coating composition increased significantly and it was no longer possible to spray coat it, and therefore the following evaluations could not be carried out.

[0196] [Table 5]

[0197] [Table 6]

[0198] [Table 7]

[0199] All of the aqueous coating compositions of the Examples were capable of forming coating films that were curable at low temperatures, had high hardness, were excellent in gloss, and produced little spray dust.

[0200] A melamine resin having a small average number of functional groups was used in Comparative Examples 1, 2, 8, and 9. It was confirmed that the coating films obtained by low-temperature curing of these aqueous coating compositions had poor hardness. A blocked isocyanate compound was used in Comparative Examples 3 and 10. It was confirmed that the coating film obtained by low-temperature curing of this aqueous coating composition had significantly inferior hardness. A non-hydrophilic isocyanate compound was used in Comparative Examples 4 and 11. It was confirmed that the coating film obtained by low-temperature curing of this aqueous coating composition was significantly inferior in gloss and generated a large amount of spray dust. In Comparative Examples 5 and 12, an aqueous coating composition was prepared by adding a melamine resin to the second liquid. The viscosity of this aqueous coating composition increased significantly, making it difficult to spray coat. No polyisocyanate compound was used in Comparative Examples 6 and 13. It was confirmed that the coating film obtained by low-temperature curing of this aqueous coating composition had significantly inferior hardness. In Comparative Example 7, a melamine resin with a large average particle size was used. The coating film obtained by low-temperature curing of this aqueous coating composition had poor gloss. This is thought to be because the melamine resin has light diffusing properties.

[0201] The present disclosure includes the following aspects. [1] A first liquid containing a hydroxyl group-containing resin (A) and a melamine resin (B); A second liquid containing a hydrophilic polyisocyanate compound (D), the hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2); the melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average total number of imino groups and methylol groups per triazine ring exceeds 1; The hydrophilic polyisocyanate compound (D) is at least one of an ionically modified polyisocyanate compound (D1) and a nonionically modified polyisocyanate compound (D2). [2] A first liquid containing a hydroxyl group-containing resin (A), a melamine resin (B) and a polyurethane resin (C); A second liquid containing a hydrophilic polyisocyanate compound (D), the hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2); the melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average total number of imino groups and methylol groups per triazine ring exceeds 1; The hydrophilic polyisocyanate compound (D) is at least one of an ionically modified polyisocyanate compound (D1) and a nonionically modified polyisocyanate compound (D2). [3] The aqueous multi-component coating composition according to the above [1] or [2], wherein the ion-modified polyisocyanate compound (D1) contains at least an anion-modified polyisocyanate compound. [4] The aqueous multi-component coating composition according to the above [2], wherein the polyurethane resin (C) has a hydroxyl value of 30 mgKOH / g or less. [5] The aqueous multi-component coating composition according to any one of the above [1] to [4], wherein the hydroxyl-containing resin (A) has an acid value of 5 mgKOH / g or more and 70 mgKOH / g or less. [6] The water-based multi-component coating composition according to any one of the above [1] to [5], wherein the melamine resin (B) has a solubility parameter of 9 or more and 15 or less. [7] The aqueous multi-component coating composition according to any one of the above [1] to [6], wherein the solid content by mass of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the solid content by mass of the hydroxyl group-containing resin (A). [8] The aqueous multi-component coating composition according to the above [2], wherein the solid content mass of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the total solid content mass of the hydroxyl group-containing resin (A) and the polyurethane resin (C). [9] The aqueous multi-component coating composition according to any one of the above [1] to [8], wherein the equivalent ratio of the imino groups and methylol groups of the melamine resin (B) to the isocyanate groups of the hydrophilic polyisocyanate compound (D) (imino groups and methylol groups / isocyanate groups) is 0.2 or more and 1.1 or less.

[10] The water-based multi-component coating composition according to any one of the above [1] to [9], wherein the melamine resin (B) has a number average molecular weight of 300 or more and 3,000 or less.

[11] A step of applying any one of the above-mentioned aqueous multi-component coating compositions [1] to

[10] onto a substrate to form an uncured first coating film; A step of applying a clear coating composition onto the uncured first coating film to form an uncured clear coating film; and curing the uncured first coating film and the uncured clear coating film by heating at 70°C or higher and 110°C or lower.

[12] A step of applying any one of the above-mentioned aqueous multi-component coating compositions [1] to

[10] onto a substrate to form an uncured first coating film; A step of applying a second aqueous coating composition onto the uncured first coating film to form an uncured second coating film; A step of applying a clear coating composition onto the uncured second coating film to form an uncured clear coating film; and heating the uncured first coating film, the uncured second coating film, and the uncured clear coating film at a temperature of 70°C or higher and 110°C or lower to cure them.

[13] The method for producing a coated article according to

[11] or

[12] above, wherein the substrate includes a metal part and a resin part. [Industrial Applicability]

[0202] According to the aqueous coating composition of the present invention, it is possible to form a coating film that can be cured at low temperature, has high hardness, has excellent gloss, and suppresses spray dust. Therefore, it is particularly suitable for use on substrates that include both metal and resin parts.

[0203] This application claims priority based on Japanese Patent Application No. 2023-013411 filed in Japan on January 31, 2023, and priority based on international application PCT / JP / 2023 / 035702 filed on September 29, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A first liquid containing a hydroxyl group-containing resin (A) and a melamine resin (B); A second liquid containing a hydrophilic polyisocyanate compound (D), The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2), the melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average total number of imino groups and methylol groups per triazine ring exceeds 1; The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a nonion-modified polyisocyanate compound (D2).

2. a first liquid containing a hydroxyl group-containing resin (A), a melamine resin (B) and a polyurethane resin (C); A second liquid containing a hydrophilic polyisocyanate compound (D), The hydroxyl-containing resin (A) is at least one of a hydroxyl-containing acrylic resin (A1) and a hydroxyl-containing polyester resin (A2), the melamine resin (B) is dissolved in the first liquid or has an average particle size of less than 1 μm, and the average total number of imino groups and methylol groups per triazine ring exceeds 1, The hydrophilic polyisocyanate compound (D) is at least one of an ion-modified polyisocyanate compound (D1) and a nonion-modified polyisocyanate compound (D2).

3. 3. The aqueous multi-component coating composition according to claim 1, wherein the ion-modified polyisocyanate compound (D1) contains at least an anion-modified polyisocyanate compound.

4. 3. The aqueous multi-component coating composition according to claim 2, wherein the polyurethane resin (C) has a hydroxyl value of 30 mgKOH / g or less.

5. 3. The aqueous multi-component coating composition according to claim 1, wherein the hydroxyl-containing resin (A) has an acid value of 5 mgKOH / g or more and 70 mgKOH / g or less.

6. 3. The aqueous multi-component coating composition according to claim 1, wherein the melamine resin (B) has a solubility parameter of 9 or more and 15 or less.

7. 3. The aqueous multi-component coating composition according to claim 1, wherein the solid content of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the solid content of the hydroxyl group-containing resin (A).

8. 3. The aqueous multi-component coating composition according to claim 2, wherein the solid content mass of the melamine resin (B) is 7 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the total solid content mass of the hydroxyl group-containing resin (A) and the polyurethane resin (C).

9. 3. The aqueous multi-component coating composition according to claim 1, wherein the equivalent ratio (imino group and methylol group / isocyanate group) of the imino group and methylol group of the melamine resin (B) to the isocyanate group of the hydrophilic polyisocyanate compound (D) is 0.2 or more and 1.1 or less.

10. 3. The aqueous multi-component coating composition according to claim 1, wherein the melamine resin (B) has a number average molecular weight of 300 or more and 3,000 or less.

11. A step of applying the aqueous multi-component coating composition according to claim 1 or 2 onto a substrate to form an uncured first coating film; A step of applying a clear coating composition onto the uncured first coating film to form an uncured clear coating film; and curing the uncured first coating film and the uncured clear coating film by heating at 70°C or higher and 110°C or lower.

12. A step of applying the aqueous multi-component coating composition according to claim 1 or 2 onto a substrate to form an uncured first coating film; A step of applying a second aqueous coating composition onto the uncured first coating film to form an uncured second coating film; A step of applying a clear coating composition onto the uncured second coating film to form an uncured clear coating film; and heating the uncured first coating film, the uncured second coating film and the uncured clear coating film at a temperature of 70°C or higher and 110°C or lower to cure them.

13. The method for producing a coated article according to claim 11, wherein the object to be coated includes a metal part and a resin part.

14. A method for manufacturing a coated article as described in claim 12, wherein the substrate includes a metal part and a resin part.